Human influenza hemagglutinin (HA) is a surface glycoprotein required for the infectivity of the human virus. The HA tag is derived from the HA-molecule corresponding to amino acids 98-106 has been extensively used as a general epitope tag in expression vectors. Many recombinant proteins have been engineered to express the HA tag, which does not appear to interfere with the bioactivity or the biodistribution of the recombinant protein. This tag facilitates the detection, isolation, and purification of the proteins.

Cystatins are natural inhibitors of papain-like (family C1) and legumain-related (family C13) cysteine peptidases. The mammalian cystatin superfamily members are of three major types, including the type 1 cystatins (stefins), type 2 cystatins and the kininogens. As a member of type 2 cystatin, cystatin D is a single-domain protein and also has cysteine residues that form disulfide bridges. In contrast with the wider distribution of all the other family 2 cystatins, cystatin D is tissue-restricted expressed and has been found only in saliva and tears. and meanwhile, it displays an inhibition profile with a preferential inhibition on cathepsin S, H, L. Although the exact functions are largely unknown, it has reported that cystatin D is involved in the inhibition of virus replication and apoptosis.